1. Field of the Invention
This invention relates to a thermodynamic system operating through a positive displacement compressor-expander device, and more particularly to a highly efficient positive displacement system.
2. Related Art
The subject invention pertains to improvements across a wide spectrum of applications in the field of thermodynamics. Therefore, an overview of the various terms and categories within the field of thermodynamics will provide a proper context for this invention. A thermodynamic system is a set of components that control the flow and balance of energy and matter in that part of the universe under consideration. Thermodynamic systems may be described as closed or open systems, as these terms are generally understood by those of skill in the art. A closed-system may be defined as a fixed mass under study. An open-system may be defined as a fixed region in space under study. An open-system will exchange mass with its surroundings, but a closed-system will not.
A cycle is a set of thermodynamic processes whose initial and final states are identical. A cycle is commonly represented in engineering practice by drawing the set of processes on pressure-volume (p-V) diagrams or temperature-entropy (T-s) diagrams. There are many common cycles used in thermodynamics including the Otto cycle, Diesel cycle, and Brayton cycle. These cycles can be used to develop both heat engines and refrigeration systems. The Carnot cycle models the most efficient cycle for a heat engine or refrigeration system.
A particular focus in the study of thermodynamics is that of energy, or the ability to do work. According to the universally understood first law of thermodynamics, the total energy of a system and its surroundings is conserved. Energy may be transferred into a thermodynamic system by heat input, work input (e.g. by compression), and/or mass input. Conversely, energy may be extracted from a thermodynamic system by heat output (cooling), work output (e.g. by expansion), and/or mass output. In the case of positive displacement pump systems, as viewed from a thermodynamics perspective, energy is transferred mechanically by force applied to a body and its resulting displacement, and through heat transfers. Systems may be designed to reduce the overall energy required to operate a thermodynamic system, leading to increased operating efficiencies, lower operational costs, and/or reduced greenhouse gas emissions.
Positive displacement type thermodynamic systems may be expressed in real life through various constructions or applications. For example, a positive displacement thermodynamic system may be embodied in a heat engine, in which combustible material is “burned” within an enclosed space and the heat energy is converted into work. In heat engines the direction of heat transfer is from high temperature to low temperature. Heat may also be moved from a lower temperature to a higher temperature in a refrigeration system by applying work. Such systems commonly function on either a thermodynamic gas cycle or vapor cycle refrigeration.
Heat engines may be classified as being either of two types: internal combustion or external combustion. There are two common types of internal combustion engines—spark ignition and compression ignition. Both may be implemented through piston/cylinder devices that typically operate on a four-stroke cycle, although other stroke combinations have been proposed. Such internal combustion engines may have one or more cylinders, each configured with an intake and exhaust manifold. Each manifold is typically fitted with a valve to control the flow of a working fluid to and from the cylinder. The operation of a compression ignition engine, such as a diesel engine, includes the following events:
The cycle begins with the piston in a “top-dead center” (TDC) position. The top-dead center is a reference to the crank position at this time. At this point, the engine will have just completed the previous cycle and is prepared to begin a new cycle.
Stroke 1—Intake (e.g., Process 1-2 in FIG. 2A): The cycle starts with the intake-valve open and the exhaust-valve closed. A crankshaft pulls the piston away from top-dead center. This creates a negative pressure in the cylinder relative to the outside air and results in a charge of fresh air being pulled into the chamber.
Stroke 2—Compression (Process 2-3 in FIG. 2A): As the crank continues to rotate, the intake-valve is shut. The piston moves back toward the top-dead center position compressing the air trapped in the cylinder. The temperature and pressure in the cylinder both rise as a result of the compression.
As the piston approaches top-dead center, a small quantity of fuel is injected into the chamber. Because of the high temperatures in the cylinder, the fuel-air mixture created by the injection spontaneously ignites, releasing the fuel's chemical energy. The temperature increases dramatically as a result. The pressure may or may not increase, depending on the manner of heat release.
Stroke 3—Expansion (Process 3-4 in FIG. 2A): The high temperature/pressure combustion gas forces the piston away from top-dead center. This stroke is often referred to as the power stroke. Near the completion of the stroke, the exhaust-valve opens and the product gases start to rush from the cylinder.
Stroke 4—Exhaust (Process 4-1 in FIG. 2A): As the crank continues to rotate, the motion of the piston continues to scavenge the exhaust gases from the cylinder. Near the completion of the stroke, the exhaust-valve closes and the intake-valve opens to prepare for the next cycle.
In a typical reciprocating piston engine, a slider-crank device attached to the piston converts the mechanical energy created during the power stroke to rotary motion. Mechanical energy leaves the engine via the rotating crankshaft. In a four-stroke engine, two rotations of the crankshaft are required to complete one cycle. Unconverted thermal energy from the combustion process leaves the engine via the escaping exhaust gas, and by a cooling fluid used to limit the engine components' maximum operating temperatures.
In these and other thermodynamic scenarios, the ratio of theoretical work output to theoretical heat input is an important parameter in engine design. This is known as thermal efficiency, something that engine designers attempt to maximize. The ratio of chamber volume at bottom-dead center to top-center (or its equivalent in rotary type devices) is known as the compression ratio. It is often said that increasing the compression ratio will increase the thermal efficiency of an engine. But such increased thermal efficiency can be obtained only so long as the increased compression ratio results directly in the capability to burn more fuel by bringing more air into the combustion chamber. This necessary condition is not sufficient to produce the expected efficiency increase without many other conditions being met. Volumetric efficiency is primary among variables including valve timing, spark and combustion timing, reaction kinetics, time available (RPM), fuel placement, distribution, atomization, etc. These all control peak temperatures and pressures actually developed as well as heat and noxious byproducts of combustion per gram of fuel. Volumetric efficiency is the foundation for all other measures because it is the ratio between the mass of fluid (air) actually delivered compared to the mass theoretically contained in the working volume at any stipulated temperature and pressure.
In conventional piston engines, the compression ratio is equal to the expansion ratio. In some non-conventional piston engines, the expansion ratio may be increased relative to the compression ratio, producing asymmetric compression and expansion processes. Such non-conventional engine designs are considered advantageous on the belief that a longer expansion can be used to extract more work from a given heat input, thereby increasing thermal efficiency.
In a typical 4-stroke internal combustion engine, the power-stroke is just one of the four strokes (hence it is available for only 180 out of 720 degrees of crank rotation). To enable smooth-running performance, energy is stored in a large and heavy flywheel during the power-stroke for release in the other 540 degrees. If a more constant power supply were available, i.e., more than a single 180 degree power-stroke per 720 degrees of crank revolution, it might be possible to reduce the size of the flywheel (or its equivalent), which would lead to a reduction in the overall size and weight of the engine—an especially important concern in mobile applications.
There are two major differences between a spark ignition and a compression ignition engine. The first difference has to do with how fuel and air are combined. In a spark ignition engine, fuel is mixed with air prior to entering the cylinder, and a spark ignites this mixture as the piston approaches top-dead center. In a compression ignition engine, on the other hand, fuel is not pre-mixed with air prior to entering the cylinder. The second difference involves how engine speed and torque is controlled. In spark ignition engines, a throttle valve constricts the air flow into the cylinder, and fuel is added to match the amount of air pulled into the cylinder. In a compression ignition engine, however, there is no throttle valve and the engine speed and torque is controlled by the amount of fuel injected into the cylinder or cylinders.
This “throttling” distinction between spark- and compression-ignitions engines is significant because the operation of a spark ignition engine is often idealized by modeling the actual cycle using a thermodynamic cycle called the Otto cycle, as shown in FIGS. 1A and 1B. The Otto cycle is represented by a reversible-adiabatic (isentropic) compression, a constant volume heat addition, isentropic expansion, and a constant volume heat removal (due to the exhaust of the combustion gas). In the model, the pressure at state 4 is higher than the pressure at state 1. This means that potential work is lost when the exhaust valve is open, as far as 45 degrees before Bottom Dead Center. A plot of pressure versus volume actually measured for a typical spark ignition engine is shown in FIG. 1C, with the area circumscribed by the curve representing the work that is done at wide-open throttle. The area enclosed on the PV trace or indicator diagram from an engine represents work done by the working fluid on the piston. The “indicated mean effective pressure,” or imep, is a measure of the indicated work output per unit swept volume, in a form independent of the size and number of cylinders in an engine or its engine speed. The indicator diagram of FIG. 1C shows the imep as a shaded area equal to the net area of the indicator diagram.
In a 4-stroke cycle, the negative work occurring during the induction and exhaust strokes is termed the pumping loss. This negative work is subtracted from the positive indicated work of the other two strokes. Returning to the “throttling” distinction between spark- and compression-ignitions engines, when an engine is throttled down from wide open throttle to the maximum speed allowed on superhighways, the pumping loss increases thereby reducing engine efficiency. Pumping losses increase dramatically beyond those shown for the common speeds of city driving. In FIG. 1C, the shaded area has the same volume scale as the indicator diagram, so the height of the shaded area must correspond to the imep. Those who are skilled in this field will appreciate that the peak pressure generated by a thermodynamic system such as an engine is commonly ten times (10×) or greater more than the mean pressure that it generates. And the mean pressure is available for only one of the four strokes, i.e., 180 degrees out of 720.
Those skilled in the art will readily appreciate that the indicator diagram is a recognized depiction of the work produced during a cycle. Tracing pressure versus piston position implies equivalence between a unit of time and a unit of piston movement. The work reflected in the cycle diagram is sometimes mistaken for a portrayal of power. Work may be shown in relation to piston position but power relates to the first derivative of piston position, work per unit time or PdV/dt. Those skilled in the art will acknowledge that when the indicator diagram's picture of work is remapped into the power domain it follows the shape of a sine wave whose value at TDC is zero. This is true in spite of the fact that the piston's linear speed is constrained by the uniform angular velocity of the flywheel throughout its stroke.
Tracing PdV as if it were an adiabat the Otto, Diesel, Dual Cycle and indicator diagrams not only hides the times at which work is available as power, but also disguises the time spent releasing heat without work. Standard analytical practices fail to measure lost thermal potential except to take note of reduced mechanical efficiency, another average which has been carved away from shaft angle. Excluding parasitic loads and friction, high speed mechanical efficiencies drop to substantially due primarily to losses described above. Industry and laboratory measures complacently accept average cycle yields rather than identify peak and actual power per degree of shaft rotation.
The prior art has long recognized the inefficiencies which exist in a real-world thermodynamic system operating on the Otto cycle, and innovators have sought to improve the efficiencies in various ways. One well-known technique to capture work otherwise lost is to enable a longer expansion stroke than the compression stroke, described earlier as asymmetric expansion-compression. One example of such a device is known as the Atkinson cycle engine. Original Atkinson cycle engines used a linkage to achieve a longer expansion stroke than compression stroke. More recent implementations of the Atkinson principal, for example those used in current production Toyota Prius vehicles, deliver the equivalent of a shorter compression stroke by bringing less air into the combustion chamber and at a lower pressure through variable valve timing. In these situations, the piston moves through the same length compression stroke and expansion stroke as used in a conventional engine, but the intake valves are left open during the initial stages of the compression stroke. Instead of compressing the air charge early in the stroke, the air is pushed back out of the open intake valve. After a short delay, the intake valve is closed and the actual compression stroke begins. This approach creates an asymmetric ratio of compression-stroke volume to exhaust-stroke volume and ensures complete recovery of the mechanical energy in the combustion gas. Unfortunately, it also results in a portion of the compression-stroke being wasted or going unused, thereby under utilizing the compressor volume and contributing to inefficiencies such as friction and heat loss. The largest penalty associated with the wasted compression stroke is the corresponding reduction in the mass of air inducted to the engine. With less air in the cylinder, less fuel can be added and less power produced. Atkinson cycle engines are known for good thermal efficiency but relatively poor power-to-volume and power-to-weight ratios.
The potential gain in work from a device such as the Atkinson cycle engine is illustrated by the highlighted regions in FIGS. 1D and 1E, in which the expansion phase is extended all the way to state 5. While this gain in work does improve the overall thermodynamic efficiency of the engine, the reduced compression volume in these engines tends to reduce the relative power output of the engine, as previously described. Therefore, current Atkinson cycle approaches have failed to achieve full thermodynamic benefits. This is also the case with other devices that have attempted to capture lost work by producing asymmetric compression and expansion processes. For reference, the volumetric efficiency of un-throttled spark ignition and diesel engines ranges from 75% to 90% according to reliable sources. The volumetric efficiency is much lower by design in methods now used to approximate an Atkinson cycle.
Compression ignition engines suffer from the same waste of mechanical energy when the exhaust valve opens before the combustion gases are expanded completely to atmospheric pressure. Compression ignition engines may be modeled using either a diesel cycle (FIG. 2A) in which the heat addition is modeled as being in constant pressure, or a dual cycle (FIG. 2B) in which the heat addition is modeled as part constant volume and part constant pressure. In either case, the efficiency can, in theory, be improved by using a longer expansion stroke than compression stroke. This increase in thermodynamic efficiency is indicated in FIGS. 2A and 2B by the added “tails” extending to respective states 5 and highlighted as potential gains. As with attempts to achieve this gain in spark ignition engines, as in FIG. 1D, attempts to capture this theoretical gain in both diesel and dual-cycle thermodynamic systems have been impractical or incomplete.
An inherently efficient gas turbine stands in contrast to the inherently inefficient positive displacement heat engines described above. As shown schematically in FIG. 3A, a gas turbine is an open flow device, but its operation can be explained in terms of the positive-displacement four-stroke engines discussed previously. A rotary compressor at the front of the turbine engine provides the intake and compression functions on a continuous stream of inlet air. Fuel is injected in the combustion chamber downstream of the compressor and ignited, releasing the energy in the fuel. The high-temperature combustion gases flow downstream to a turbine, which provides the expansion and exhaust function. The expanding gases through the turbine turn a set of blades which allows a path for mechanical energy to leave the engine. The operation of a gas turbine can be represented by the schematics shown in FIGS. 3A and 3B. The arrangement of FIG. 3A is referred to as open-loop and the arrangement of FIG. 3B is referred to as closed-loop. An idealization of this process is the Brayton cycle as shown in FIGS. 3C and 3D. The compression process (from 1 to 2) is modeled as being isentropic, the heat addition (from 2 to 3) is constant pressure, and the expansion through the turbine (from 3 to 4) is isentropic. The heat removal process (from 4 to 1) may be accomplished in one of two ways: by rejecting the exhaust gas, as shown in FIG. 3A, which is an open thermodynamic cycle also known as an open-loop system; or by using a heat exchanger, as shown in FIG. 3B, a closed cycle or closed-loop system.
In the Brayton cycle, the recovery of energy from the combustion gas is complete, since the expansion (from 3 to 4) is to atmospheric pressure. Note that the expansion process in FIGS. 3C and 3D defines a substantially greater volume than the compression process. This asymmetric ratio is required in order for complete energy recovery to occur. The compressor blade design sets the compression ratio (rc=V1/V2) of the rotary compressor, or alternatively the pressure ratio (rp=P2/P1). The expansion ratio for the turbine (re=V4/V3) is determined by thermodynamic relationships dependent on the compression ratio and the amount of heat added in the process from 2 to 3.
Turbine-based thermodynamic systems are highly efficient at recovering energy and operating at nearly ideal conditions. However, turbine-based thermodynamic systems are not well suited to low speed and highly variable operating conditions. As a result, turbine-based thermodynamic systems and engines are not typically used for automotive transportation and other such systems in which variable loads are common.
Moving away from heat engines, another type of thermodynamic system that can be implemented through a positive displacement compressor-expander device is refrigeration. Instead of extracting work from the movement of heat from a higher temper to a lower temperature (a heat engine) a refrigerator uses work to move heat from a lower temperature to a higher temperature. Just as heat engines implemented through positive displacement compressor-expander devices are plagued by low-efficiency issues, refrigeration systems face similar problems.
Broadly defined, refrigeration systems may be operated to provide targeted cooling or heating. The term “heat pump” is gaining prominence as an inclusive term for refrigeration because it more generically describes the process of moving heat from a low temperature to a higher temperature by supplying mechanical work. A heat pump integrated with an air conditioner is a refrigeration system that can be used to heat a home as well as cool it. In both heating and cooling modes it may be configured to use the same permanently installed inside and outside heat exchangers, with the direction of heat flow merely reversed.
A common method for refrigeration is based on the vapor-compression cycle as shown in FIGS. 4A and 4B. The refrigerant changes phase as it passes through the components. One may observe that the direction of travel (shown with notations 1, 2, 3, 4) through this cycle is reversed from the heat engines shown in the preceding figures. An evaporator is stationed in the region to be cooled. Saturated vapor exits the evaporator at state 1, and is then compressed to an elevated pressure and temperature at state 2. As the refrigerant passes through the condenser, heat is transferred to the surrounding atmosphere and the refrigerant is condensed to a saturated liquid. The refrigerant is then passed through an expansion (or throttling) valve, where it flashes into a mixture of liquid vapor. The resulting low-temperature, low-pressure refrigerant then passes through the evaporator absorbing heat from the refrigerated space. The ability to do work is lost when passing through the expansion valve from high pressure at state 3 to low pressure at state 4. An alternative to the expansion valve would be to expand through an energy conversion device, which would bring the refrigerant to state 4′, as illustrated in the Ts diagram of FIG. 4B. If accomplished, this would have two positive effects—it would reduce the net work into the unit and also increase the amount of energy that could be absorbed in the evaporator. Both of these effects would be expected to improve the coefficient of performance for the refrigerator. However, while these benefits have been theoretically forecast, practical units have not been constructed due to the difficulty of design and construction, and with the constraints of providing a low-cost energy recovery device that is sufficiently reliable.
Referring to FIGS. 5A and 5B, it is also well known that the Brayton cycle, as discussed previously, may be used to develop a refrigeration system. When the Brayton cycle is operated as a refrigeration system, the cycle is run in the opposite direction, counter-clockwise instead of clockwise. If air is used as a working fluid, it will not undergo a phase change, as it would in vapor-compression refrigeration. In an air cycle refrigeration device, an expander naturally recovers the available energy in the working fluid as it passes from state 3 to state 4, as illustrated in FIGS. 5A and 5B. However, practical attempts to implement this kind of theoretical system have relied on turbine-based systems in which the compression ratio used by the device is fixed by blade geometry. Furthermore, constraints of high volume throughput and steady state operation also limit applications for this technology to certain, very specified and limited settings only. As a result, if the refrigeration unit is producing too much cold air, the unit is cycled on and off to maintain proper temperatures. A more efficient approach would be to only make as much cold air as is needed. At reduced cooling (or heating) loads, significant economies would result from bringing heat exchanger temperatures closer to both indoor and outdoor temperatures, thereby also reducing the temperature difference (lift) the system would have to deliver. Present turbine-based systems are not practically suited to achieve this type of highly efficient operation.
Accordingly, there is a need in the art to provide an improved thermodynamic system which is positive displacement structured rather than turbine-based, and which is capable of achieving highly efficient operation whether configured as a power system or a refrigeration system, and of maintaining its efficiency at low operating speeds and under variable load conditions.